The ability of polymers to act as electrical insulators is the basis for their widespread use in the electrical and electronic fields. However, material designers have sought to combine the fabrication versatility of polymers with many of the electrical properties of metals. There are instances when an increased conductivity or relative permittivity of the polymer is warranted, such as in applications which require antistatic materials, low-temperature heaters, electromagnetic radiation shielding and electric field grading. A few select polymers, such as polyacetylene, polyaniline, polypyrrole and others, can be induced to exhibit intrinsic conductivity through doping, though these systems tend to be cost prohibitive and difficult to fabricate into articles. Furthermore, polymer composites have been fabricated which exhibit favorable conductivity characteristics. However, those composites which exhibit extreme immiscibility between their minor and major phase materials have inherently poor mechanical properties and, as engineering materials, are not useful for most applications.
Percolation theory is relatively successful in modeling the general conductivity characteristics of conducting polymer composite (CPC) materials by predicting the convergence of conducting particles to distances at which the transfer of charge carriers between them becomes probable. The percolation threshold (p.sub.c), which is the level at which a minor phase material is just sufficiently incorporated volumetrically into a major phase material resulting in both phases being co-continuous, that is, the lowest concentration of conducting particles needed to form continuous conducting chains when incorporated into another material, can be determined from the experimentally determined dependence of conductivity of the CPC material on the filler concentration. For a general discussion on percolation theory, see the October 1973 Vol. 45, No. 4, Review of Modern Physics article, entitled, Percolation and Conduction, the contents of which are herein incorporated by reference. Much work has been done on determining the parameters influencing the percolation threshold with regard to the conductive filler material. See for example, Models Proposed to Explain the Electrical Conductivity of Mixtures Made of Conductive and Insulating Materials, 1993 Journal of Materials Science, Vol. 28; Resistivity of Filled Electrically Conductive Crosslinked Polyethylene, 1984 Journal of Applied Polymer Science, Vol. 29; and Electron Transport Processes in Conductor-Filled Polymers, 1983 Polymer Engineering and Science Vol. 23, No. 1; the contents of each of which are herein incorporated by reference. See also, Multiple Percolation in Conducting Polymer Blends, 1993 Macromolecules Vol. 26, which discusses "double percolation", the contents of which are also herein incorporated by reference.
Attempts for the reduction of conductive filler content in CPC materials have been reported for polyethylene/polystyrene and for polypropylene/polyamide, both employing carbon black as the conductive filler. See for example, Design of Electrical Conductive Composites: Key role of the Morphology on the Electrical Properties of Carbon Black Filled Polymer Blends, 1995 Macromolecules, Vol. 28 No. 5 and Conductive Polymer Blends with Low Carbon Black Loading. Polypropylene/Polyamide, 1996 Polymer Engineering and Science, Vol. 36, No. 10, the contents of both of which are herein incorporated by reference.